1. Field of the Invention
The invention relates to a data processing method for an inkjet printing apparatus that prints an image by ejecting a printing liquid such as ink on a print medium while a print head scans. In particular, the invention relates to a data processing method of an inkjet printing apparatus that prints an image on a print medium by way of bi-directional print scans of a plurality of print heads that eject different inks.
2. Description of the Related Art
As for serial type color inkjet printing apparatuses there are many cases where multiple print heads, corresponding to ink colors such as yellow (Y), magenta (M), cyan (C), and black (K), for example, are mounted on a carriage such as to be aligned in the direction of movement of the carriage. An image is gradually formed on the print medium by way of alternately repeating print scans that eject ink from these print heads according to image data while the carriage moves, and conveyance operations that convey the print medium in a sub-scanning direction that crosses the direction of print scanning.
Here, if a bi-directional printing is performed that ejects ink in both the forward scanning and backward scanning of the carriage it is possible to decrease the printing time in comparison to a one-direction printing that ejects ink during the forward scans only. With such bi-directional printing, however, the order of applying ink on the print medium during the forward scans and backward scans is reversed.
For example, in the case where print heads are arranged in the carriage in the order K, C, M, and then Y in the main scanning direction, during the forward scan ink is applied on the print medium in the order K, C, M, and then Y. During the backward scan, however, ink is applied in the order Y, M, C, and then K. That is, bands in which ink is applied in the order K, C, M, and then Y and bands in which ink is applied in the order Y, M, C, and then K are disposed alternatingly on the print medium along the sub-scanning direction. It is known that generally, in the case where multiple colors of ink are applied on a print medium, although the ink combination is the same, when the order of application differs a difference in coloring appears. Hence even in the case of a uniform image a color difference between the bans printed at the forward scans and the bands printed at the backward scans occurs, and this brings forth an image deterioration known as color unevenness.
Such color unevenness can be reduced to an extent by performing a multi-pass printing, for example. In a multi-pass printing, printing is performed with respect to only a portion of the pixels, among the plurality of pixels disposed in 1 band, during a single print scan. Next, printing is performed with respect to the remaining pixels at separate, differing print scans. Thus, multiple pixels at which ink was applied in the order K, C, M, and then Y, and multiple pixels at which ink was applied in the order Y, M, C, and then K, are mixed together at all bands. As a result it is difficult for a color difference to stand out between adjacent bands.
The sizes of the individual dots that are formed on the print medium, however, are generally designed to be larger than the area of 1 pixel. Thus, as is to be expected, even in the case where multi-pass printing has been performed a color difference will occur to some extent between bands at which printing was performed in the order K, C, M, Y then Y, M, C, K and bands at which printing was performed in the order Y, M, C, K then K, C, M, Y.
In Japanese Patent Laid-Open No. 2001-180017, for example, a configuration is disclosed wherein attention is given to the existence of images where color unevenness stands out evenly and images where color unevenness is difficult to stand out, and in which bi-directional printing and one-directional printing are switched according to the image data. More concretely, the number of dots printed at a prescribed area of the print medium are counted by ink color, bi-directional printing is performed at areas at which the counted result does not exceed a threshold value that is feared for color unevenness, and one-directional printing is performed at areas where the counted result exceeds the threshold value. If such a printing method is employed, at images where color unevenness is difficult to stand out an image is printed at a comparatively high speed by a bi-directional printing, and it is possible to prevent the occurrence of color unevenness by way of a one-directional printing at images where color unevenness stands out easily.
With the method of Japanese Patent. Laid-Open No. 2001-180017, however, in the case of images where areas exceeding the prescribed threshold continue in succession, a printing operation that is the same as a conventional one-directional printing will be performed and it is not possible to obtain the high speed output effect of bi-directional printing.